1. Field of the Invention
The present invention relates to a compound semiconductor substrate having a hetero-junction which is best suited for a high electron mobility field-effect transistor (HEMT) that uses two-dimensionally quantized electrons.
2. Description of the Related Art
To improve the performance of a transistor, a mechanism is necessary for switching as many electrons as quickly as possible. In general, when donor impurities are added in large amounts to a semiconductor layer in order to increase the concentration of electrons, the impurities are scattered so greatly that the mobility of electrons decreases inside the semiconductor. This problem can be solved by a known high electron mobility field-effect transistor (HEMT) which uses a material of a high mobility such as GaAs or the like and an active layer of a two-dimensional electron gas (2DEG).
The HEMT has a feature that the layer (channel layer) in which the electrons run and the layer (doping layer) for supplying electrons are composed of different materials, and that the electrons are confined in the channel layer as a two-dimensional electron gas due to the quantum effect, are isolated from the donor impurities in the doping layer, and are scattered slightly. In order to further decrease the scattering, there is often inserted between the doping layer and the channel layer a spacer layer composed of the same material and having the same composition as the doping layer but having no impurities.
Performance of the HEMT can be effectively improved by enhancing the mobility of the channel layer. Recently, a study has been carried out on forming the channel layer by using InGaAs having a mobility greater than that of GaAs. Also, AlGaAs or InAlAs have been known materials for the doping layer that can be used in combination with the InGaAs channel layer.
Here, InGaAs exhibits such a property that the mobility enhances with an increase in the In content (In composition ratio). Increasing the In composition ratio, therefore, makes it possible to obtain a transistor having high performance resulting, however, in an increase in the lattice constant which causes lattice mismatching with respect to the doping layer and the substrate material. Therefore, a pseudomorphic HEMT obtained by growing crystals under a pseudomorphic state is now attracting attention. This utilizes the fact that crystals of good quality can be grown without disorder in the lattice, such as dislocation, even when the materials having different lattice constants are grown involving lattice mismatching though the crystal lattice is distorted and deformed, provided the thicknesses of the films are smaller than a predetermined value called critical film thickness. That is, by increasing the In composition ratio of the channel layer and by selecting the In composition ratio of the doping layer to be smaller than that of the channel layer, a difference in the band gap can be increased between the InGaAs channel layer and the doping layer, whereby the quantum effect efficiently works to confine more electrons making it possible to accomplish both an increase in electron concentration and an increase in mobility.
Here, the confinement of electrons by the quantum effect is determined by the shape of the quantum well that is established by the potential of the conduction band. In order to theoretically analyze the quantum effect, the distribution of two-dimensional electron gas (2DEG) accumulated in the channel layer is calculated by a numerical calculation method which solves the Schraedinger equation and Poisson's equation in a self-consistent analysis. This calculation method is the one that is used for finding the electron distribution in the HEMT.
FIG. 6 shows calculated results of a potential profile and a distribution of 2DEG of the pseudomorphic HEMT of a conventional structure which comprises, formed on a semi-insulating InP substrate 1 in the order mentioned by the crystal growth, an InAlAs buffer layer 2 having an In composition ratio of 52% and whose lattice matches the substrate 1, an InGaAs channel layer 3 having an In composition ratio of 80% and is pseudomorphic to the buffer layer 2, an InAlAs spacer layer 5 for feeding carriers and having an In composition ratio of 52%, an n-type InAlAs doping layer 6 having an In composition ratio of 52% doped with donor impurities, and an n-type InGaAs cap layer 7 having an In composition ratio of 53% which prevents the doping layer from being oxidized.
With the structure which is capable of confining many electrons such as pseudomorphic HEMT as will be obvious from FIG. 6, the potential of the channel layer (InGaAs) 3 is sharply curved and is greatly recessed near the hetero-interface. Therefore, the distribution of electron density exhibits a sharp peak near the hetero-interface and the electrons exist in a large proportion near the hetero-interface.
On the other hand, donor impurities exist in the doping layer (InAlAs) 6 and remain therein being positively charged after the electrons are emitted, and a Coulomb force is applied to the electrons in the channel layer 3. Therefore, the electrons exhibiting a peak deflected toward the hetero-interface are subject to be greatly scattered as they approach the donor impurities; i.e., Coulomb scattering increases relative to the doping layer and the characteristics are adversely affected.
In the pseudomorphic HEMT, furthermore, the channel layer has a lattice constant which is different from those of other layers and receives distortion. As mentioned above, therefore, the channel layer must have a thickness which is smaller than a critical film thickness. Even when the channel layer has a thickness which is smaller than a critical thickness, however, there still remains a problem in that the stability is lost due to aging or due to a sudden change in the temperature, and resistance anisotropy is observed in which the resistivity changes depending upon the crystal azimuth. Moreover, distortion imparted to the film increases as the thickness of the layer to which the distortion is imparted increases, and the above-mentioned problem becomes more conspicuous.
The problem in that the electrons are concentrated near the hetero-interface can be solved by technology according to which a very thin InAs layer is inserted in the channel layer, and the distribution of 2DEG is located at a position remote from the hetero-interface by utilizing the fact that InAs has a small band gap with respect to InGaAs as described in, for example, a Report ED90-115 of the Japanese Association of Electronic Information Communication. However, there still remains a problem in that greater distortion is imparted to the channel layer since InAs has a lattice constant which is larger than that of InGaAs.